Design for radar-automatic frequency control mixer



H. H. ARNLD Filed Dec. 2.0, 1950 ,ya f r W M ,Z W Z A y u a l1 M m m .WWM. m im m f/J w E ,u ,431 i .llIl/L .l`ll V 7^ N. 2 ri Q IILJ w l a f WM2 n m n n m vn u Aug. 10, 1954 DESIGN FOR RADAR-AUTOMATICFREQUENCYCONTROL MIXER Patented Aug. 10, 1954 UNITED STATES FTENT OFFICE DESIGNFOR RADAR-AUTOMATIC- FREQUENCY CONTROL MIXER Cl. Z50- 20) 4 Claims.

This invention relates to a device for preventing energy cross couplingbetween adjacent mixer stages in a hollow wave guide and moreparticularly to a unitary hollow wave guide mixer assembly having meansfor preventing energy cross coupling between a radar mixer and an AFCmixer.

In the design and construction of radar transmitter-receiver assemblies,it is often necessary to place both a radar mixer and an AFC mixerwithin a single hollow wave guide so that both mixers may be energizedfrom a single local oscillator. However, the use of two mixers within asingle section of wave guide is not too successful becausg the AFC mixerfrequently becomes overloaded and consequently produces a large amountof voltage at a harmonic of the desired AFC frequency. Experimentationhas determined that this overloading of the AFC crystal mixer is causedby extraneous coupling of the transmitted radar pulse from the radarmixer through the hollow wave guide to the AFC mixer. The presence ofthis large amount of harmonic frequency voltage limits the sensitivitywhich may be attained by the AFC unit since the harmonic voltage causesa high sensitivity AFC unit to lock in at the harmonic frequency and notat the desired fundamental frequency of the radar transmitter. Since theamount of harmonic frequency voltage present varies considerably amongeach of a plurality of radar transmitter-receiver units, it isimpossible to design a standard AFC unit which will have maximumsensitivity and also be capable of use with all transmitter-re- -ceiverunits. An attempt to overcome this limitation by providing a variablesensitivity control on each AFC unit was unsuccessful because of thedifficulties of establishing a proper sensitivity adjustment under fieldservice conditions.

Accordingly, an object of this invention is to prevent extraneous orcross coupling of oscillatory electrical energy between two adjacentmixers which are positioned within a single section of hollow waveguide.

Another object of this invention is to provide a unitary radarAFC mixerassembly having a single local oscillator for supplying oscillatoryelectrical energy to a radar mixer and an AFC mixer wherein crosscoupling of the energy between the two mixers is prevented whilepermitting a free flow of oscillatory energy from the local oscillatorto each of the mixers.

In view of these and other objects, one embodiment of the inventioncomprises a unitary radar- AFC mixer assembly having a wave guide stubprojecting upwardly from a hollow wave guide adjacent a local oscillatorand between a radar mixer crystal within the wave guide. and an AFCmixer crystal within the wave guide so as to prevent cross coupling ofoscillatory energy between the crystals without attenuating the energysupplied to each of the crystals from the local oscillator.

Many other objects and advantages of this invention will be apparentfrom a consideration of the description in conjunction with the drawingswherein Fig. l is a plan view of a unitary radar-AFC mixer assemblyembodying the invention;

Fig. 2 is an elevational View of the radar-AFC mixer assembly;

Fig. 3 is a partial sectional view, taken along line 3 3 in Fig. l,showing the relative positions of a radar mixer crystal and an AFC mixercrystal with respect to a wave guide stub;

Fig. 4 is a perspective View of a portion of the wave guide showingschematically the iiow of energy from a local oscillator to each of themixer crystals; and

Fig. 5 is a perspective view of a portion of the wave guide illustratingthe manner in which the terminated wave guide stub prevents a flow ofenergy between the mixer crystals.

Referring now to the several gures of the drawing wherein like referencenumbers indicate the same elements throughout the several views, andmore particularly tor Fig. 1 of the drawing, a straight hollow waveguide I0 is provided with a plurality of hollow wave guide sections ortransmission branches II, I2, I3, and I4 which extend perpendicularlyoutward from the wave guide I0. One end of the wave guide l0 isapertured and is provided with an integral flange i5 which is used tointerconnect the wave guide I0 with an external beacon cavity 3l,

The hollow wave guide section or transmission branch II has an integralflange I6 which serves to interconnect the branch I I with a localbeacon oscillator 32 so that oscillatory energy from the local beaconoscillator 32 is supplied to the wave guide It. A connecting flange I'Ion transmission branch I2 interconnects the signal voltage from atransmitting-receiving tube 33 to the interior of the hollow wave guideIt.

A ange I8 on transmission branch I4 interconnects an external AFC signalsource 34- withy the branch I4 so that the control voltage from the AFC'signal source 311 is coupled to the interior of wave guide III. Thetransmission branch. I3 is connected to an external local. oscillator 30by a connecting ange I9 so that oscillatory energy is supplied fromoscillator 30 to a pair of crystal mixers 2i) and 2l.

Crystal mixer 20 is secured to and extends through the upper wall ofwave guide I approximately midway along the transverse dimension of thewave guide l@ and in alignment with the transmission branch i2. Crystalmixer 2U serves as the radar mixer and is energized by oscillatingvoltage from the local oscillator 3G connected to branch i3 and from thesignal input voltage supplied through branch I2.

Crystal mixer 2! is secured to and extends through the upper wall ofWave guide li! at a position midway along the transverse dimension ofwave guide i@ and in alignment with transmission branch Eil. Crystal 2lis the AFC mixer and is supplied with oscillatory energy from the localoscillator 33 connected to branch i3 and from AFC unit 3c connected towave guide section i4.

A hollow wave guide stub 22 which is terminated by an energy absorbingclosure 'E3 extends perpendicularly upward from the upper wall of waveguide l in alignment with transmission branch l3- and at a positionapproximately midway between the mixers 2G and 2i. This position of stub22 with respect to the mixers 2li and 2i is not critical. The terminatedstub 22 is matched in impedance with the remainder of the Wave guideassembly to prevent signal distortion due to reflected waves from stub22. One pair of side walls 25 of stub 22 is parallel to the side wallsof transmission branch I3 and preferably is almost equal in length tothe transverse dimension of the upper wall of wave guide iii. Anotherpair of side walls 2l of stub 22 is parallel to the side walls of waveguide l0 and preferably is shorter in length than the transversedimension of the upper wall of transmission branch i3.

The schematic diagrams in Figs. 4 and 5 illustrate how wave guide stub22 prevents the undesired cross coupling of energy past mixer 22 andthrough wave guide iii to mixer 2l while permitting a free flow ofenergy from the local oscillator 363 to each of the mixers 2S and 2|. InFig. 4, local oscillator 3i) supplies energy to both of the mixers il@and 2i through branch i3 and wave guide l@ in the manner shown byvectors 2li if a TEoi mode of propagation is assumed. Since theattenuating wave guide stub 22, shown in dotted outline, does notintercept the vectors 213 indicating the flow of energy to the mixers2B' and 2i, there is no appreciable attenuation of the energy flowing toeach of the mixers 28 and 2i from local oscillator 3E).

However, as shown in Fig. 5, vectors 25, which indicate the extraneouslyor cross coupled energy feeding from mixer 23 to mixer 2l, are almostcompletely intercepted by the stub 22, shown in dotted outline, so thatmost of the undesired cross coupled energy is absorbed in the stub dueto attenuation. By thus preventing the transfer of energy from mixer 20to mixer 2i, the overloading of the AFC crystal mixer 2i is positivelyprevented together with the attendant undesired production of harmonicfrequency voltage.

It is to be understood that the above described embodiment is merelyillustrative of the principles of this invention and that numerous othermodifications may readily be devised by those 4 skilled in the art whichwill fall within the spirit and scope of these principles.

What is claimed is:

l. A hollow rectangular wave guide assembly comprising a main waveguide, first and second mixers longitudinally spaced and mounted on andextending through a top wall of the main wave guide, a rst wave guidebranch extending from a first side wall of said main wave guide andbetween said first and second mixers to feed oscillator energy thereto,a second wave guide branch extending from a second side Wall of the mainwave guide and in alignment with the first mixer to feed oscillatoryenergy thereto, a third wave guide branch extending from said secondside wall in alignment with said second mixer to feed oscillatory energythereto, a fourth wave guide branch extending from said first side walladjacent said rst mixer to feed oscillatory energy thereto, and aterminated wave guide stub tending through said top wall of the mainwave guide between said rst and second mixers to prevent energy couplingtherebetween while permitting energy flow to said mixers from the firstbranch.

2. A hollow wave guide transmission system for protecting an automaticfrequency control (AFC) mixer against overload comprising a radar mixerand the automatic frequency control mixer spaced from each other withina hollow wave guide, a nrst transmission branch extending outwardly fromthe wave guide between the two nixers for feeding local oscillatoryelectrical energy thereto, a second transmission branch of the waveguide in alignment with the radar mixer for feeding electrical energy ofa first frequency thereto, a third transmission branch of the wave guidein alignment with the AFC mixer for feeding electrical energy of asecond frequency thereto, a fourth transmission branch of the wave guideadjacent the radar mixer for feeding electrical energy of a thirdfrequency to the radar mixer, a fth transmission branch at one end ofthe wave guide and adjacent the radar mixer and connected to a beaconcavity, and a hollow wave guide stub connected to the wave guide betweenthe two mixers and in alignment with the rst transmission branch toprevent the transmission of electrical energy between the two mixers,said stub being matched in impedance with the remainder of the waveguide assembly.

3. in a radar system, a hollow main wave guide, a radar mixer, anautomatic frequency control mixer, said mixers mounted in spacedrelation in the upper surface of said main wave guide and centrallylocated with respect to the lateral dimension of the upper surface ofthe main wave guide, a rst hollow wave guide branch perpendicularlysecured to said main wave guide, said first branch being in alignmentwith the automatic frequency control mixer, an automatic frequencycontrol signal source connected to said first branch to energize theautomatic frequency control mixer, a second hollow Wave guide branchperpendicularly secured to said main wave guide, said second branchbeing in alignment with the radar mixer, a transmitting-receiving tubeconnected to said second wave guide branch to connect a received signalto the radar mixer, a third hollow wave guide branch perpendicular-lysecured to said main wave guide, said third branch secured to a side ofthe main wave guide opposite that of the irst and second branches andthe axis of the third branch being centrally located between the axes ofthe first and second branches,

a source of oscillatory energy connected to the third branch forenergizing both the radar mixer and the automatic frequency controlmixer, and a hollow wave guide stub connected to the upper surface ofthe main Wave guide between the two mixers and in alignment with thethird wave guide branch to prevent the transfer of electrical energybetween the two mixers, said stub being matched in impedance with theremainder of the wave guide assembly and terminated by an energyabsorbing closure.

4. 1n a radar system, a wave guide assembly comprising a hollowrectangular main wave guide having two horizontal sides and two verticalsides, an automatic frequency control mixer mounted on the upperhorizontal side of the main wave guide and extending therethrough, aradar mixer mounted similarly to and spaced from said automaticfrequency control mixer, a first transmission branch connectedperpendicularly to the main wave guide and in alignment with theautomatic frequency control mixer to guide energy from an automaticfrequency control signal source to the automatic frequency controlmixer, a second transmission branch connected perpendicularly to themain wave guide and in alignment with the radar mixer to guide areceived signal from a transmitting-receiving tube to the radar mixer, athird transmission branch connected perpendicularly to the main waveguide and centrally disposed between the mixers to guide energy from alocal oscillator to both of said mixers, a fourth transmission branchconnected perpendicularly to the main Wave guide and adjacent the radarmixer to guide energy from a beacon oscillator to the radar mixer, saidrst and second transmission branches extending from an opposite verticalside of the main wave guide than said third and fourth transmissionbranches, a fth transmission branch extending from an end of the mainwave guide and adjacent the radar mixer to guide energy from the radarmixer to a beacon cavity, and a hollow wave guide stub having four sidesmounted on the upper horizontal side of the main wave guide and inalignment with the third transmission branch, two of the sides of thestub being parallel to and slightly less in length than the horizontalsides of the main wave guide and the other two sides of the stub beingparallel to and slightly less in length than the lateral dimension ofthe third transmission branch, said stub being matched in impedance withthe remainder of the Wave guide assembly and terminated with an energyabsorbing material to prevent the cross-coupling of energy from theradar mixer to the automatic frequency control mixer.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,413,939 Benware Jan. 7, 1947 2,496,521 Dicke Feb. 7, 19502,514,678 Southworth July 11, 1950 2,545,994 Gabler et al Mar. 20, 19512,577,540 Pound Dec. 4, 1951 2,593,120 Dicke Apr. 15, 1952

